Our invention relates to Venetian Blinds, and, more particularly to monocontrol Venetian Blinds that use the same operating control both for controlling the tilting of the slats, and for raising and lowering the blind.
The headrail mechanism of a Venetian blind must provide for two operations; first, lifting and lowering the blind, and second, controllably tilting the slats to open or close the blind or set the slats at any desired angle. The ideal monocontrol headrail mechanism would require low operation effort, even when lifting heavy and long blinds. It would provide for accumulating the lift cords or tapes within a relatively small headrail. It would have a tilt mechanism capable of providing good closure. And finally, it would contain a minimum of parts and be easy to assemble and require a minimum of adjustment.
The prior art does contain a number of designs for monocontrol Venetian blinds. Some of them do not tilt sufficiently to provide good closure. Many of them use a large number of complex parts and are difficult to adjust.
The prior art reveals two general methods for accumulating lift cords or tapes within a Venetian blind headrail. One method is to wind the lift cords or tapes onto spools. This method suffers two disadvantages. One disadvantage is that the cords or tapes do not wind evenly onto their respective spools, and very slight differences in diameter produce easily noticeable unevenness in the blind as it is raised. The other disadvantage is that the mechanical advantage of the lift mechanism decreases as the diameter of the accumulated cord or tape increases on the spools. This progressive decrease in mechanical advantage occurs as the lift cords support more of the blind's weight, causing a large increase in the effort required to further lift the blind. The mechanical advantage decreases just when it should increase.
The other, and preferred method is to accumulate the cords onto a shaft that moves laterally, or traverses, so that the lift cords wind in a single layer onto the shaft. This insures even winding of each of the lift cords, and it maintains a constant mechanical advantage so that the lifting effort increases only in proportion to the weight supported by the lift cords. Several methods have been used to produce the traversing of the rod. A rack and gear arrangement has been used. U.S. Pat. No. 1,343,527 reveals a lead screw and nut to accomplish the traversing. Another method, one that is free of any gears or leadscrews, is revealed in U.S. Pat. No. 4,625,012 in which the lift cords, acting on cam features of their supporting cradles, produce the lateral forces to traverse the rod. Although, this method is presently used in a variety of blinds that lift from the bottom, it was not believed that the method could work with Venetian blinds because it was expected that the weight of the blinds hanging on the tilters would add so much frictional drag that the rod would not traverse properly.
The headrail mechanism must also provide for tilting the slats of a Venetian blind. In a monocontrol blind that employs a traversing rod on which to accumulate the lift cords, the tilt mechanism must rotate in either direction along with the traversing rod until the position for full closure is reached. Thereafter, the tilt mechanism must slip, maintaining its position, while the blind is raised or lowered. In a traversing rod monocontrol Venetial blind, the drive shaft for the tilt mechanism is the traversing rod. The ladder cords are attached to the tilter mechanism. Generally, in a monocontrol blind, there will be one tilter mechanism for each ladder cord in the blind. The best tilting results if the ladder cords are attached to the tilter at a separation equal to the width of the slats. Furthermore, a line between these attachment points should pass through the centerline of the traversing rod. This will keep the tilter at the same angle as the slats. If this geometric relationship is not maintained, then it will be necessary for the tilt mechanism to be capable of lifting the blind if full closure is to be achieved.
In a Venetian blind having separate lift and tilt mechanisms, there is no difficulty in providing a tilt mechanism capable of lifting the weight of the entire blind. However, in a traversing rod monocontrol blind, the traversing rod must drive both the tilt and the lift mechanisms. When the fully tilted position is reached, the tilt mechanism must partially disengage and slip, providing, thereafter, sufficient torque to maintain full tilt as the rod continues to rotate for lifting or lowering the blind. Whatever force is needed to maintain full tilt is added to the effort required to lift the blind. This force will be minimum if, (a) the tilter geometry is as described above, and (b) if the tilter mechanism is capable of adjusting its grip on the traversing rod to provide only that amount of torque needed to maintain full tilt. Furthermore, the smaller the separation of the two sides of the ladder cords at full tilt, the better the closure will be. This last requirement will be best satisfied if the tilters are mounted directly on the traversing rod. Any intermediate part between the tilters and the rod will increase the separation of the ladder cords at full tilt. U.S. Pat. No. 4,697,630 reveals a tilter mechanism which has good gripping torque between extremes of tilt while partially releasing its grip when full tilt is reached. This tilter is made in the form of a multiturn helical band clutch which grips the traversing rod during tilting. When the position for full tilt has been reached, the leading end of the tilter contacts a stop which prevents further movement of the tilter and partially releases the grip of the tilter on the rod, thereby limiting the frictional drag of the tilters on the rod to just that amount of torque required to maintain the fully tilted condition of the blind. The rod can continue to rotate, winding or unwinding the lift cords to raise or lower the blind according to the direction in which the rod is being rotated.
U.S. Pat. Nos. 2,737,235, 2,758,644, and 3,352,349, describe prior art monocontrol Venetian blinds that employ a variety of traversing rod lift mechanisms. In order to achieve traversing in each of the prior art blinds it is necessary to overcome all of the frictional force due to the weight of the blind acting on the traversing rod. The grip of the tilt mechanism on the traversing rod must be sufficient to ensure complete closure. And, of course, both the tilt force and the drag on the rod must be overcome to cause the rod to traverse. These frictional forces are large enough to make these blinds very difficult to operate. It is, therefore, not surprising that Venetial blinds using this type of mechanism have never been popular.